1. Field of the Invention
The present invention relates to plasma generation apparatus and use of plasma generation apparatus. The invention has particular, but not exclusive, application to the sterilization of goods and/or surface decontamination.
2. Related Art
It is known that electrical discharges may be produced deliberately in order to provide useful effects. It is known, for example, to produce ozone (O3) using a corona discharge in air at atmospheric pressure. For example, U.S. Pat. No. 3,967,131 discloses a corona discharge ozone generating unit in which a corona discharge is generated between concentric electrodes separated by an air gap and a cylindrical dielectric (glass) shield and connected to an ac voltage of about 12,000 volts. The electrodes are formed from stainless steel mesh.
A corona discharge is a form of plasma, i.e. a partially ionized gas, including a proportion of free electrons and ions, including excited neutral species.
The production of ozone is of interest in particular for sterilization and water-treatment applications, since ozone can render harmless micro organisms such as bacteria, and yet itself is unstable and degrades to oxygen in a relatively short time. Sterilization using ozone has many potential advantages over competing technologies such as heating, chemical disinfection, UV radiation and nuclear radiation.
U.S. Pat. No. 6,007,770 discloses an apparatus and method for sterilizing objects held in a closed vessel, such as in a sealed plastic bag. The apparatus has two parallel high voltage electrodes facing each other via two parallel insulators, there being a gap between the two parallel insulators. The closed vessel is placed in the gap between the parallel insulators. Operation of the apparatus causes oxygen in the closed vessel to be converted to ozone. The electrodes are operated at a voltage of 7-25 kV.
U.S. Pat. No. 6,007,770 discloses an embodiment in which there is an internal electric conductor in the closed vessel. This allows a lower applied voltage to be used. However, this document also explains that where the closed vessel is narrow, there may be no need for an internal electric conductor in the closed vessel.
It is considered that the operation of the device in U.S. Pat. No. 6,007,770 would generate ozone outside the closed vessel, resulting in a significant operator hazard. Furthermore, where no internal electric conductor is located in the closed vessel, the closed vessel must have a very small lateral extent (e.g. up to about 10 mm) in order to limit the applied voltage. Even so, a high voltage must be used, causing a very high electric field across the closed vessel, which will have the result of damaging sensitive items, including items containing electronics components. Furthermore, when the vessel includes conductive items (for example most foodstuffs), the arrangement of those conductive items will affect the amount of ozone produced, and can cause hotspots that damage the vessel.
US 2003/0108460 discloses a method and apparatus for producing a surface corona discharge. A base electrode and a mesh electrode are separated by a dielectric spacer. The mesh electrode is grounded. An ac voltage of 2.5-3.6 kV at 60 Hz is applied to the base electrode to produce a surface corona discharge for the production of ozone. The corona discharge is formed in the openings in the mesh electrode. US 2003/0108460 discloses the sterilization of objects in a plastic bag, such as tissues, organs, food products, etc. In this case, a lid of the plastic bag is formed with the ozone-producing apparatus, separated from the remainder of the volume of the plastic bag by a porous dielectric plate. Ozone produced by the ozone-producing apparatus disperses to the objects in the plastic bag through the porous dielectric plate. Thus, it is a requirement of US 2003/0108460 that there is no seal between the ozone-producing apparatus and the objects in the plastic bag, otherwise ozone cannot reach the objects in the plastic bag.
Schwabedissen et al 2007 [A. Schwabedissen, P Lacinski, X. Chen and J. Engemann, “PlasmaLabel—a new method to disinfect goods inside a closed package using dielectric barrier discharges” Contrib. Plasma Phys. 47 No. 7, 551-558 (2007)] disclose a ozone-based sterilization procedure. WO 2006/039883 provides a similar disclosure.
Schwabedissen et al disclose the generation of an atmospheric pressure plasma inside a sealed package by relying on a surface dielectric barrier discharge supported by a label on the inner surface of the package. The label includes a metallic electrode, here designated the inner electrode. An outer electrode arrangement is placed in contact with the outer surface of the package, and a high voltage ac signal is applied to the outer electrode arrangement. The outer electrode arrangement is capacitively coupled to the inner electrode, thereby driving a surface dielectric barrier discharge inside the sealed package and generating ozone.
The inner electrode used by Schwabedissen et al has a complex shape. This is in order to maximise the inner electrode edge length on the label, since the electric field strength is high at these edges. The metal of the inner electrode faces the interior of the package.